1. Field of the Invention
The present invention relates to purging, and in particular to purging pellicle volumes in lithography.
2. Related Art
In lithography, a reticle is often protected by a pellicle. A pellicle is mounted on a stand-off (also called a pellicle frame) from the reticle. The space enclosed by a reticle, a pellicle frame, and a pellicle, is called the pellicle volume. In some lithographic applications, such as 157 nm wavelength exposure applications, a pellicle volume needs to be purged and filled with a certain quality of a certain gas before the reticle can be exposed. For example, in 157 nanometer (nm) applications, nitrogen (pure or nearly pure) is often used to fill a pellicle volume before a reticle can be exposed for printing patterns on wafers.
To accomplish purging of a pellicle volume, gas can passively diffuse into the pellicle volume space. Such passive gas diffusion, however, takes a long time to achieve a required gas concentration and purity in the pellicle volume. Active purging, where gas is directed to a pellicle volume, has been done at low pressure levels which takes a relatively long time to purge a given volume. Such delay reduces productivity and increases the overall lithographic manufacturing process time. Active purges have been limited to low pressures to protect a pellicle. Faster purging of a pellicle volume requires higher pressures that can exert excessive stress on a pellicle.
A pellicle can be made out of hard or soft materials. Hard pellicles are often made out of glass. Soft pellicles are often made out of organic membranes or other flexible materials. Regardless of material type, pellicles are often thin. As a result, many pellicles are vulnerable to stress—especially due to a displacement force on a plane of the pellicle. Fast purging of a pellicle volume at high pressure can exert excessive stress on a pellicle causing damage and possibly rupture. This problem even limits fast purging of flexible pellicles since they may be stretched and unable to return to their original shape.
Fast purging is made even more difficult by the poor permeability of many pellicle frames. Pellicle frames have been made of materials which are solid or nearly impermeable to gas flow to prevent particles from entering the pellicle volume. Impermeability to gas makes solid pellicle frames unsuited for applications where a pellicle volume must be purged. Gas-permeable materials are increasingly being used in pellicle frames to allow the pellicle volume to be purged. Such gas-permeable pellicle frames are still somewhat restrictive to purging flows, since the material must also act as a particle barrier. As a result, higher pressures must be applied to purge a pellicle volume quickly.
What is needed is a method and system for fast purging of pellicle volumes in lithography that can quickly and reliably purge pellicle volumes without causing undue stress or damage to a pellicle.